A substrate is a relatively flat and rigid structure that provides mechanical support for a die in an IC package and typically transmits signals and power to and from the IC. A conventional substrate often includes conductive leads that are connected to the die so that the die exchanges signals with other dice and/or circuits in the IC package in addition to other circuits that may be connected to the IC package. IC packages are often applied to circuit board assemblies that are part of systems of interconnected IC packages which form electronic devices such as computers or cellular phones.
One method of bonding a die to a substrate in an IC package is flip-chip bonding. One version of flip-chip bonding is known as the controlled collapse chip connection, or C4 method. The C4 method typically includes placing solder bumps on bonding pads that are exposed on the dice. The solder bumps are usually placed on the dice before the dice are separated from the wafer. Each die is then turned over (i.e., flipped) and aligned with a corresponding pattern of bonding pads or solder bumps on a substrate. A reflow procedure is carried out to join the bumps/pads on the substrate and die to form a series of solder columns between the die and the substrate. The solder columns serve as conductive paths between the die and the substrate through which signals are transmitted and power is delivered.
Substrates are typically mounted onto a board, such as a motherboard. One method of forming the interconnects between the substrate and the board is called ball grid array (BGA). In a BGA, an array of solder balls (larger than the C4 solder connection) is melted and formed on the substrate. The entire substrate is then inverted onto the surface of the board to connect the BGA to the board by again melting the solder to form solder joints.
The pads on most types of substrates are typically made of copper that is covered with a surface finish before solder is added to form the pads which are connected to a die. Electroless nickel and immersion gold are presently used as the surface finish for C4 and BGA pads in high density interconnect substrates.
The electroless nickel provides a barrier that is critical in reducing electro-migration between the copper and the solder that form the pads. Electro-migration can cause reliability problems in pads, especially in high current applications where there may be unacceptable levels of electro-migration between the copper and solder.
The immersion gold provides an oxidation barrier to the electroless nickel. The oxidation barrier is crucial because nickel oxide readily forms on electroless nickel and it is difficult to bond any material, especially solder, to thick nickel oxide.
There are reliability concerns associated with using electroless nickel as a barrier because phosphorous ends up in the electroless nickel when the electroless nickel is applied to the copper pad. The phosphorous ends up in the electroless nickel that is formed on the pads because of the manner in which electroless nickel is applied to the pad.
An alternative to applying an electroless nickel and immersion gold surface finish on pads in high density interconnect (HDI) substrates is to apply direct immersion gold right onto the copper pads. A direct immersion gold surface finish provides enhanced toughness of the solder joint/pad interface.
One drawback associated with using a direct immersion gold surface finish is that it does not include a barrier. Therefore, a direct immersion gold surface finish may not be reliable due to electro-migration, particularly in high-powered CPU applications which involve higher current and temperatures for extended periods of time.
HDI substrates typically utilize solder resists that are patterned to form openings over the pads and/or interconnects. The openings over the pads and/or interconnects are adapted to receive the solder that forms the finished pads which are attached to the die. The solder resists need to be strongly bonded directly to the surface finish of the bonding pads so that a strong joint can be formed between the copper and the solder. Therefore, using direct immersion gold as surface finish for the pads causes another concern because it is extremely difficult for the solder resist material to be bonded directly to a gold surface.
In the FIGS., like reference numbers refer to like elements.